1. Field of Invention
This invention relates to a method of manufacturing a display device using an electro-optic substance such as liquid crystal. In particular, it relates to a method of manufacturing an active matrix substrate having driver elements and pixel electrodes arrayed in a matrix.
2. Description of Related Art
In a display device using an electro-optic substance such as a liquid crystal, the electro-optic substance is generally enclosed in a gap between two substrates. On one of the substrates, electrodes corresponding to pixels (pixel electrodes) are arrayed in a matrix form. By applying a voltage to the pixel electrodes, optical characteristics of the electro-optic substance located in the region corresponding to each of the pixels are changed so as to display an image.
An active matrix display is especially provided with an active matrix substrate such that a driver element such as, for instance, a transistor, is arranged along with the pixel electrodes for applying a predetermined voltage to the pixel electrode.
There are two types of display devices: a transmission-type display device and a reflection-type display device. The transmission-type display device has a transparent substrate, such as a glass plate, on which a thin semiconductor layer is formed so as to form a thin film transistors (TFT) as the driver element. The pixel electrode may also be formed of a transparent material.
On the other hand, the reflection-type display device may have an opaque substrate. For example, a reflection-type display device is formed on a semiconductor substrate (a silicon wafer). Driver elements, such as transistors, are formed on the surface of the substrate itself or on a thin semiconductor layer formed on the surface of the substrate. A pixel electrode is formed of an opaque material with a high reflectivity such as, for instance, an aluminum film.
An example of the active matrix substrate used in the transmission-type display device is shown in a plan view of FIG. 11. The active matrix substrate shown in FIG. 11 includes a pixel region 102 having several tens to several millions of pixels formed thereon in a matrix arrangement, a black matrix (light shield region) 104, and a wiring region 106. Each portion of the pixel region 102 shielded by the black matrix is provided with a TFT for switching an electric charge entering and leaving each pixel electrode.
FIG. 12 is a schematic sectional view showing a principal structure of an active matrix liquid crystal display device using the active matrix substrate shown in FIG. 11.
The liquid crystal display device shown in FIG. 12 includes a transparent substrate 110 shown at the bottom of the drawing, and a TFT active layer (thin semiconductor layer) 112 formed on the surface of the substrate 110. A gate electrode 114, a data line 116, a drain electrode 118, an interlayer dielectric layer 120, and a black matrix (light shield region) 104, are formed above the TFT active layer 112. On the interlayer dielectric layer 120, a pixel electrode 122a made of a film of, for instance, transparent conductive material such as ITO (indium tin oxide) and an alignment film 124a formed of, for example, polyimide are formed.
The stack of layers in the lower part of the display device, from the substrate 110 to the alignment film 124a, makes up the active matrix substrate 130. On the other hand, as shown in the upper part of the drawing, on a transparent substrate (upper substrate) 128, an electrode 122b made of a transparent conductive film and an alignment film 124b are formed so as to constitute an opposing substrate 132.
The active matrix substrate 130 and the opposing substrate 132 are held together so as to oppose each other, forming a space (cell gap) G therebetween. Liquid crystal 126 is injected into the cell gap G.
An even cell gap G is preferable, because uneven cell gap between the substrates causes unevenness in the brightness and color of the display. In practice, however, it is difficult to realize a high degree of accuracy of the cell gap G over the entire surface of the display device.
One of the reasons is that the active matrix substrate 130 and the opposing substrate 132 are not completely flat. In the active matrix substrate 130 in particular, various films are deposited with various degrees of stress and are thermally treated at various temperatures. Even if the substrate 110 is completely flat, it is difficult to maintain the final active matrix substrate 130 flat.
In order to control the evenness of the cell gap G, a method is known in which particulate spacers (beads) are sprayed on the active matrix substrate. See, for example, Japanese Unexamined Patent Application Publication No. A-10-104636.
Also, a technique is proposed to provide cell gap holding members (columnar spacers) by patterning a film of resin or inorganic material on the active matrix substrate. For example, Japanese Unexamined Patent Application Publication No. A-10-339889 proposes a method to form gap holding members by patterning a photosensitive polyimide film planarized by spin coating and standing at room temperature and further planarized by CMP (chemical mechanical polishing). Further, Japanese Unexamined Patent Application Publication No. A-8-248425 proposes a method to form the spacers by patterning a silicon oxide film deposited by plasma CVD (chemical vapor deposition) and planarized by the CMP method.
In the method of spraying particulate spacers, as schematically shown in FIG. 13, however, particulate spacers 134 are arranged at random positions. In other words, the positions of the particulate spacers 134 cannot be controlled. The particulate spacers 134 arranged in the pixel region 102 disturb the orientation of the liquid crystal, causing distortion in the displayed image.
As a countermeasure therefor, Japanese Unexamined Patent Application Publication No. A-10-104636 proposes a method of blowing air to the spacers after they are sprayed so that the spacers remain only in regions other than the pixel region. Even when this measure is employed, however, the particulate spacers move during the process of injecting the liquid crystal into the cell gap. Therefore, it is difficult to completely control the positions of the particulate spacers in practice. Also, the movement of the spacers during liquid crystal injection causes dispersion in the density of spacers on the surface. This dispersion in the density increases the dispersion in the cell gap.
On the other hand, in a method of forming columnar spacers by patterning resin or inorganic material film, the spacer position is controllable. However, resin materials generally have poor thermal and light stability. Shrinkage of the columnar spacers due to thermal cycles causes dispersion in the cell gap. Also, the image quality may be degraded by irradiation with light.
In the case where the columnar spacers are formed with inorganic material, the thermal and light stability can be improved. However, as shown in FIG. 11, the surface on which an inorganic film is deposited by plasma CVD has unevenness due to the presence of the pixel electrodes and the TFTs. The surface of a silicon oxide film deposited on such a substrate is hence not flat due to the unevenness of the surface of the substrate. If such a film is patterned, flat-top columnar spacers with even height cannot be formed, and a constant cell gap cannot be maintained.
Accordingly, Japanese Unexamined Patent Application Publication No. A-8-248425 proposes to planarize the surface of a silicon oxide film by CMP. However, as will be described in detail later, it is difficult to achieve a high flatness by the CMP of the entire surface of the film.